| Suppose that you have a 7200/240V 10KVA transformer with 4% impedance.
| It would have an impedance seen from the primary of 207 ohms (4% of 5184
| ohms). It's rated current (primary) would be 1.4A and its short circuit
| current would be 35A
| Now, at 480V the short circuit current would be 480/207 =2.32A primary which
| would be 66%ABOVE its rating (corresponding to (480/7200)*35) meaning that
| magic smoke won't appear quite as soon as it would at 35A-but it still
| would occur. The open circuit voltage would be 16V as you indicate. You
| would have to limit the input voltage to 290V in order to stay within the
| 1.4A rating.
| Changing the transformer base voltage doesn't change the actual impedance.
| if you rerated this transformer to 480V, the rated current would still be
| 1.4A (unless you want to let magic smoke out-admittedly not as soon as at
| 35A short circuit current) and the % impedance on the new base would be
| higher (60%) .
OK, maybe I have to take this a step further ... 240 volts in, 8 volts out.
| If you stacked a bunch in series as you indicated, then the performance
| would be extremely poor. Is this what you want?
What is the difference between one transformer rated 15000 VA, 7200 volts
primary, 240 volts secondary, 4% impedance ... and ... 30 transformers rated
500 VA, 240 volts primary, 8 volts secondary, 4% impedance, wired with both
primary and secondary in series as I've described (besides the latter being
a rats nest).
How many primary winding turns would the former need to have? How many
would the latter need to have? Would the latter times 30 equal the former?
One of the things that got me thinking on this was seeing a pole pig near
a road, dedicated powing a few lights on a sign. I figured there was not
more than a couple KW of light there, if that. The pole pig was rather
small. Yet I was trying to imagine what kind of primary winding it would
have given that it was probably being supplied with no less than 7200 volts.
And I was thinking that, as the KVA rating goes down, number of turns goes
up, and at voltages of 7200 and up, that's going to soon reach a point where
the size can't get smaller because of the heavier insulation needed with
the many turns (though I suppose if very carefully designed, not so much
insulation would be needed between adjacent turns).
| I don't see what you expect to gain. A stack such as this is
| performance-wise and economically, inferior to a properly designed single
And it may not be the economical way even for what I have in mind, which
is to power a load which will approach being a short circuit, such as an
electric arc. But I am only thinking about this in a theoretical sense
because I did not believe it when someone told me it was not possible to
make a transformer that would convert a voltage, limit the current, and do
this within it's load handling capacity, without using an additional pure
inductance in series with the secondary. Well, I think it certainly is
possible. Maybe what they meant is: it is not ecomically practical.
How much will the power meter spin if the load side terminals are bolted
together in a short circuit (before magic smoke gets out)? I would think
not much if any because there's no appreciable voltage. Depending on the
fault current taking place, it can still dissipate some heat, perhaps quite
fast. It would be an interesting race to see who lets the magic smoke out
first: the meter, the wiring, or the transformer.
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
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